Creativity Software

GSM Location Based Services: Example Message Flow

The message flow between network nodes involved in location services for GSM deployment is illustrated below.

Each of the steps involved is described in the paragraphs which follow. The flow corresponds to the implementation of 3GPP TS 09.31 for the Lb interface with BSSMAP-LE PERFORM LOCATION REQUEST and BSSMAP-LE PERFORM LOCATION RESPONSE primarily over SCCP for TA positioning method.

In the example case, it has been assumed that RxLev values (Signal Strengths from serving and neighbouring cells) are made available in the message through the Lb interface to enable CITARX (Enhanced Cell ID) positioning method. If Rxlevs are not available, the location calculator reverts to CITA method.

Message flow between network nodes to support location services.

1) The external LCS client makes a request (1) for the current location of a target UE from a GMLC. The GMLC verifies the identity of the LCS client and its subscription to the LCS service requested. It then derives the MSISDN or IMSI of the target UE to be located and the LCS QoS from either subscription data or data supplied by the LCS client.

2) The GMLC then (2) sends a MAP_SEND_ROUTING_INFO_FOR_LCS message to the home HLR of the target UE to be located with either the IMSI or MSISDN of the UE.

3) The HLR verifies that the SCCP calling party address of the GMLC, corresponds to a known GSM network element that is authorized to request UE location information. The HLR returns (3) the current SMSC address and either the IMSI or MSISDN if not provided in step 2) for the particular UE.

4) The GMLC (4) sends a MAP_PROVIDE_SUBSCRIBER_LOCATION message to the SMSC indicated by the HLR. This message carries the type of location information requested (e.g. current location), the UE subscriber's IMSI, LCS QoS information (e.g. accuracy, response time),  and an indication of whether the LCS client has any override capability. For a call related location request, the message also carries the LCS client's called party number. For a value added LCS client, the message carries the client name if available and, for a call unrelated location request, the identity of the LCS client. In other cases, inclusion of the client name and/or identity is optional.

5), 6) and 7) are performed by the MSC in order to page the UE and collect the location information

8) The SMSC sends (8) the BSSMAP PERFORM LOCATION message to the serving BSC for the target UE.

9) The BSC forwards (9) the BSSMAP-LE PERFORM LOCATION request received in step 5) to the SMLC. The BSC may add additional measurement data to the message to assist with positioning. The message is transported inside an SCCP connection request.

10) If the requested location information and the location accuracy within the QoS can be satisfied by the reported cell ID and, if available, TA value and signal strength/s, the SMLC sends an AP_PERFORM_LOCATION ack.  If the position method returns adequate position measurements, the SMLC uses them to compute a location estimate. If there has been a failure to obtain sufficient information to derive an approximate location estimate, an already computed location  estimate is returned for a UE based position method. If the location estimate does not satisfy the required accuracy or the location attempt fails, e.g. due to missing data, and sufficient response time still remains, the SMLC instigates a further location attempt.

11) When location information best satisfying the requested location type and QoS has been obtained, the SMLC returns it (11) to the serving BSC.

12) The BSC forwards (12) a BSSMAP PERFORM LOCATION response received in step 8) to the SMSC.

13) The SMSC returns (13) the location information to the GMLC.

14) The GMLC forwards (14) the UE location information to the requesting LCS client.

The interaction of messages between the various network nodes involved in location services to support the project is outlined in the figure below.

General interaction of messages between the various network nodes involved in location services.